Immunoreactive glandular kallikrein has been detected in rat plasma using a radioimmunoassay (RIA) for rat urinary kallikrein (RUK). Gel filtration of rat plasma revealed three peaks of antigen. Only the third peak (Peak 3) showed immunological identity the RUK. Bilateral nephrectomy significantly (p less than .001) increased the antigenic contents of whole plasma and of Peak 3. No significant difference in either whole plasma antigen or gel filtration profile was seen when pancreatectomized rats were compared to sham-operated controls, but both whole plasma antigen and the antigenic content of Peak 3 were significantly reduced (p less than .001) after bilateral submandibular/sublingual gland excision. Peak 3 was indistinguishable from purified rat glandular kallikreins on gel filtration. Purified Peak 3, however, had no activity against kininogen or the chromogenic substrate S-2266, and showed a consistently smaller molecular weight (29,500) on SDS-polyacrylamide gel electrophoresis than that of RUK (32,000). Hence, glandular kallikrein circulates in rat plasma in an inactive form. The submandibular and sublingual glands appear to be a major source of the plasma antigen and the kidney plays an important role in the clearance and/or metabolism of glandular kallikrein from plasma.